Pour the broth into a pot and keep it warm over low heat.
In a large pan, heat the olive oil with 1 tbsp butter. Sauté the onion until soft, about 3 minutes. Add the garlic and cook 30 seconds.
Add the mushrooms and cook until golden and the released liquid has evaporated, about 5 to 6 minutes.
Add the rice and toast for 1 to 2 minutes, until the edges look translucent.
Pour in the wine and stir until absorbed.
Add the hot broth one ladle at a time, stirring often and waiting until each addition is absorbed before adding more, about 18 to 20 minutes, until the rice is creamy and al dente.
Stir in the spinach and cook until just wilted, about 1 minute.
Remove from the heat. Stir in the Parmesan and remaining 1 tbsp butter. Season with salt and pepper, rest 2 minutes, and serve.
Notes
Store in an airtight container for up to 3 days. Reheat with a splash of broth to restore creaminess. Turn leftovers into crispy arancini.